Papers by Keyword: Socket Weld

Abstract: Socket welds are commonly used to assemble small-bore carbon steel process piping systems because they can be fabricated fairly quickly and are somewhat tolerant of field fit-up issues. In the most simplistic terms, these welds are made by inserting the pipe into a socket and then seal welding using gas tungsten arc welding around the gap between the outside pipe wall and the end of the fitting. This leaves a notch in the root of the seal weld that is open to the process fluid. This notch can act both as a stress riser and a crevice capable of concentrating chemical species. In many process applications, these socket weld notches do not cause any in-service problems. However, in the case of sour service, these notches can become problematic because of the potential for sulphide stress corrosion cracking (SSCC). For SSCC to occur, the right combination of stress, environment, and material susceptibility must be present. Therefore, for socket welds to be used in sour service without the risk of failure, these parameters need to be controlled by proper design and fabrication of the welded joint. Since a crevice is created as a natural by-product of a socket weld geometry, where the SSCC environment will be present, the parameter most readily controlled to minimize susceptibility to SSCC is the microstructure and residual stress of the weld filler metal. As will be shown, this resistance to SSCC can be accomplished by designing a welding procedure specification to include the combination of both a second pass seal weld and by post-weld heat treatment (PWHT). It will also be shown that a complete second pass is crucial to proper tempering and refining the microstructure of the root pass. Without this microstructural tempering and refining by the second pass, the root pass can still undergo SSCC even with subsequent application of PWHT. Examples of SSCC initiating from crack-like weld root defects caused by poor welding techniques and propagating into single-pass socket welds that had been subjected to PWHT will be presented.

Abstract: The ASME B & PV Code Sec. allows the socket weld for the nuclear piping in spite of the weakness on the weld integrity. Recently, the integrity of the socket weld is regarded as a safety concern in nuclear power plants because many failures and leaks have been reported in the socket weld. OPDE (OECD Piping Failure Data Exchange) database lists 108 socket weld failures among 2,399 nuclear piping failure cases during 1970 to 2001. Eleven failures in the socket weld were also reported in Korean NPPs. Many failure cases showed that the root cause of the failure is the fatigue and the gap requirement for the socket weld given in ASME Code was not satisfied. The purpose of this paper is to evaluate the fatigue crack behavior of a surface crack in the socket weld under fatigue loading condition considering the gap effect. Three-dimensional finite element analysis was performed to estimate the fatigue crack behavior of the surface crack. Three types of loading conditions such as the deflection due to vibration, the pressure transient ranging from P=0 to 15.51MPa, and the thermal transient ranging from T=25oC to 288oC were considered. The results are as follows; 1) The socket weld is susceptible to the vibration where the vibration levels exceed the requirement in the ASME Operation and Maintenance (OM) Code. 2) The effect of pressure or Temperature transient load on the socket weld integrity is not significant. 3) No-gap condition gives very high possibility of the crack initiation at the socket weld under vibration loading condition. 4) For the specific systems having the vibration condition to exceed the requirement in the ASME Code OM and/or the transient loading condition from P=0 and T=25oC to P=15.51MPa and T=288oC, radiographic examination to examine the gap during the construction stage is recommended.